1. Field of the Invention
The present invention relates to a stepping motor structure, a stepping motor device and a driving method thereof to rotate a rotary polygon mirror for measuring the inter-vehicle distance, direction and relative speed of a preceding vehicle.
2. Description of Related Art
Conventionally, a stepping motor using a permanent magnet for a rotor has been often used for driving a rotary portion of an office machine such as a printer, high-speed facsimile equipment, a copying machine for PPC (Plain Paper Copier) or the like, since the stepping motor has a high efficiency. Moreover, there has been a recently growing interest in preventive safety technologies to prevent traffic accidents from occurring by making vehicles more intelligent. One such preventive safety technologies to be realized is driving environment recognition, specifically inter-vehicle distance control technology using laser radar and image recognition technology. A device for such a technology requires a laser scanner using a stepping motor and a rotary polygon mirror.
A two-phase stepping motor is mainly employed for use requiring a medium accuracy, while a three-phase stepping motor excellent in cost performance is employed for use requiring high-accuracy, low vibration and low noise. As a stepping motor for office machines such as a laser printer and facsimile equipment or the like requiring accuracy in positioning and little unevenness in rotation, a three-phase machine has been often employed in order to obtain high resolution and high torque. The three-phase machine comprises a cylindrical permanent magnet type rotor formed with multiple magnets in a cylindrical shape, or a hybrid type rotor having a permanent magnet held between two magnetic plates formed with multiple pole teeth, and a stator formed with pole teeth opposite the rotor surface.
The stepping motor having the permanent magnet rotor was accurately step-driven one step angle at a time by a driving pulse input from an outer part and an output shaft of the motor rotated as if intermittently driven. Moreover, there was a growing tendency to use rare earth magnets for the permanent magnet to be employed as a rotor in order to obtain higher driving torque. The permanent magnet is magnetized with different poles alternately in strips circumferentially and the magnet surface magnetic flux density measured along the circumference of the magnet was shown as a substantially trapezoidal distribution. Further, a trapezoidal or triangular surface of the magnetic pole teeth of a stator yoke is often employed in order to obtain high torque.
The three-phase machine provided with the cylindrical permanent magnet type rotor or the hybrid type rotor and the stator formed with pole teeth is capable of obtaining high resolution and high torque as mentioned above. Since the distribution of the surface magnetic flux density of the magnetized permanent magnet is substantially trapezoidal circumferentially, step-like driving is easily achieved when the output torque is increased. On the other hand, it also has drawbacks such as increased vibration upon driving and stopping the rotor and it makes smooth driving difficult. That is, noise or vibration is generated by a vibration torque component contained in the torque generated by the product of excitation electric currents and field magnetic flux density. Accordingly, the above-mentioned construction wherein a large number of harmonics are contained in the field magnetic flux density generated in an air gap between the permanent magnet of the rotor and the stator increases noise and vibration.
Publications of Japanese Patent Application Nos. Hei 05-221388 and Hei 09-325197 disclose a permanent magnet type stepping motor to lower damping of a rotor in starting or stopping a motor by solving such problems and rotating the rotor smoothly.
According to the publication of Japanese Patent Application No. Hei 05-221388, the permanent magnet of the rotor is skew-magnetized, the magnetic flux distribution in the magnetized surface of the permanent magnet is made to be substantially sinusoidal circumferentially and the magnetic pole teeth of a stator yoke are made to be rectangular.
Moreover, according to the publication of Japanese Patent Application No. Hei 09-325197, the relation between the number of stator poles Q and the number of S-pole and N-pole pairs of the rotor N is Q=6k, and are set so as to satisfy N=yk (6n±1), and the magnetic poles of the stator are excited in a two-phase/three-phase excitation mode.
The stepping motor in accordance with each of the publications as mentioned above, however, has the following problems. Although the magnetized state of the rotor in a substantially sinusoidal shape circumferentially is to lower vibration and noise in Japanese Patent Application No. Hei 05-221388, the permanent magnet has an outer diameter configuration in which a slightly uneven cross-section which continues in each of the magnetic poles and is furthermore skew-magnetized, which makes manufacturing difficult. Moreover, the publication only discloses that a suitable number of magnetic poles (N-pole, S-pole) are skew-magnetized, but it does not describe the number of poles.
According to the publication Patent Application No. Hei 05-221388, the relation between the number of the stator poles and the number of pairs of rotors is set so that the magnetic poles of the stator are excited in a two-phase/three-phase excitation mode; however, the magnetized state of the rotor is not described.
Further, when there is a steep load change of a rotating shaft and the rotation stops due to step out, the stepping motor according to each of the above-mentioned publications remains in the stopped state. As a result, excessive current flows when the resistance value of stator windings is small without back electromotive force being generated. The flow of the excessive current increases the temperature of an adhesive or the like fixing the windings, which may cause problems such as poor insulation.